1. Field of the Invention
The present invention relates to a high intensity discharge, lamp and more particularly to an electrodeless high intensity discharge lamp of the type having an excitation coil situated about the arc tube thereof.
2. Description of the Related Art
An electrodeless high intensity discharge lamp, which is operated by a radio frequency energy, is disclosed in U.S. Pat. No. 5,151,633. The disclosed discharge lamp has an arc tube fabricated from fused quartz, or optically transparent or translucent ceramic in which a discharge medium such as ionizable gas, mercury or sodium is filled. A coil is situated about the arc tube for causing a toroidal arc discharge in the arc tube. The discharge medium emits visible radiation upon excitation of high frequency energy supplied through the coil. The arc tube fabricated from fused quartz has a small projection formed thereon. The projection which is called as an exhaust tip has been made after exhausting impurities from the arc tube and filling the discharge medium in the arc tube through an opening provided on the arc tube. The exhaust tip is formed by partially melting the arc tube when the opening is closed or sealed. When the arc tube is fabricated from ceramic which is more heat resistive than fused quartz, a ceramic cap is required for sealing the arc tube with a suitable sealing material. The sealing material is exposed with a highly activated discharge medium and is subject to damage the seal between the arc tube and the cap.
The exhaust tip or the ceramic cap exhibits high temperature during lamp operation because of heat radiation from the arc discharge. The gaseous pressure of the medium in the arc tube is determined by the temperature of the coolest part or the cold spot of the arc tube. The cold spot of the above mentioned discharge lamp moves each time when the discharge lamp is operated and also varies in temperature, which results in undesired illuminating characteristics including color spread.